1. Field of the Invention
The present invention relates to-an image forming apparatus for forming an image according to an electrographic process, such as a copier, a printer, and a facsimile machine. The present invention further relates to a process cartridge used in the image forming apparatus.
2. Discussion of the Background Art
As a charging device used in the image forming apparatus according to the electrographic process, there is known a contact-type charging roller which contacts to charge a surface of an image carrying member, such as a photoconductor. According to the contact-type charging roller, foreign materials such as toner left unremoved after transferring operations and paper particles originating from transfer sheets, which are generated from a long time use of the image forming apparatus, escape from a photoconductor cleaning device and adhere to a surface of the charging roller. Also, paper particles, toner, dust, and so forth floating inside the image forming apparatus gradually adhere to the surface of the charging roller. Such foreign materials adhered to the surface of the charging roller changes the resistance value of the charging roller. In some cases, therefore, an abnormal image is formed due to defective charging, such as uneven charging and insufficient charging.
There is also known a proximity-type charging roller which charges the surface of the photoconductor while keeping a minute gap of a few to dozens of microns with the surface of the photoconductor. The proximity-type charging roller is also subject to the change in resistance due to adhesion of the foreign materials to its surface caused over the long time use. In many cases, the proximity-type charging roller applies an AC (alternating current) voltage to the surface of the photoconductor to prevent the uneven charging. This application of the AC voltage, however, encourages adhesion of the foreign materials such as the paper particles, the toner, and the dusts floating in the image forming apparatus, to the charging roller. Therefore, similarly to the case of the contact-type charging roller, an abnormal image is formed due to defective charging, such as uneven charging and insufficient charging.
To remove the foreign materials adhered to the surface of the charging roller, there is known a charging roller provided with a charging roller cleaning member which rubs the surface of the charging roller and cleans the toner, the paper particles, and the like from the surface of the charging roller. For example, there is a charging roller cleaning roller which rubs the surface of a charging roller and cleans the toner, the paper particles, and the like from the surface of the charging roller, while being rotated along with rotation of the charging roller in contact with the surface of the charging roller. This type of charging roller cleaning roller has the advantage of higher durability, compared with a fixed charging roller cleaning member, such as a cleaning pad.
In this case, the charging roller is placed vertically above the photoconductor, and the charging roller cleaning roller is placed so as to contact the charging roller due to its own weight and to rotate along with the rotation of the charging roller. This arrangement of the charging roller cleaning roller is hereinafter referred to as upper arrangement.
Because of a spatial limit within the image forming apparatus, however, there is a case in which the charging roller cleaning roller contacts the surface of the charging roller at a position below a hypothetical horizontal plane including a rotation axis of the charging roller. In this case, the charging roller cleaning roller cannot contact the charging roller due to the weight thereof. In view of this, the charging roller cleaning roller is caused to contact the charging roller by predetermined pressure applied to a shaft thereof. In a background image forming apparatus, a transferring unit is placed vertically above photoconductors, while charging rollers are placed below the photoconductors. The charging roller cleaning rollers are provided vertically below the respective charging rollers. This arrangement of the charging roller cleaning rollers is hereinafter referred to as a lower arrangement. The lower arrangement, however, has the following disadvantage relating to the charging rollers and the charging roller cleaning rollers.
In this lower arrangement, the shaft of the charging roller cleaning roller is applied with the predetermined pressure such that the charging roller cleaning roller contacts the charging roller. Thus, extra loads are imposed on the charging roller and the charging roller cleaning roller, compared with the upper arrangement in which the charging roller cleaning roller contacts the charging roller due to the weight thereof. If the charging roller and the charging roller cleaning roller continue to be used for a long term under this state, the surface of either one of the charging roller and the charging roller cleaning roller is deteriorated. As a result, the charging roller cleaning roller fails to rotate along with the rotation of the charging roller, and thus cleaning operation of the charging roller cleaning roller becomes unstable, and cleaning performance of the charging roller cleaning roller is deteriorated. For example, the surface of the photoconductor is often coated with a lubricant to protect the surface of the photoconductor from possible hazard caused by the charging. If the lubricant adheres to the surface of the charging roller over time, the friction coefficient of a contact area where the charging roller contacts the charging roller cleaning roller substantially decreases, and the rotation of the charging roller cleaning roller along with the rotation of the charging roller becomes unstable. On the other hand, if the pressure applied for causing the charging roller cleaning roller to contact the charging roller is set to a relatively high value to maintain frictional force, the friction load on a shaft bearing increases, and the rotation of the charging roller cleaning roller is prevented. In particular, materials such as the lubricant and a toner additive easily adheres to the charging roller. Therefore, if the predetermined pressure applied to the shaft of the charging roller cleaning roller is increased, the charging roller cleaning roller rubs the materials onto the charging roller and causes filming of the materials. Thus, resistance of the surface of the charging roller becomes uneven. As a result, an abnormal image tends to be formed due to defective charging, such uneven charging and insufficient charging.